Recently, as various digital media, such as smart cards, portable terminals, electronic cashes, digital cameras, game memory, MP3 players and the like have been increasingly used, the amount of information to be processed and stored has increased rapidly, and thus the demand for various kinds of memory devices has also increased rapidly. Further, as technologies for processing a large amount of information at high speed have been increasingly required, there has been much research effort into the development of next-generation memory devices. Next-generation memory devices must process an ultra-large amount of information at high speed at low power consumption, and must be nonvolatile memory devices in which the recorded information does not disappear even after power has been turned off. To date, most research into nonvolatile memories has been done into silicon-based flash memories, but silicon-based flash memories involve some basic limitations. For example, conventional flash memories are confronted with such limitations as the number of times that recording and removing can be done is limited, the recording speed is slow, the manufacturing cost of a memory chip increases due to a miniaturization process for obtaining highly-integrated memory capacity, and a chip cannot be miniaturized any more due to physical characteristics.
As such, as conventional flash memory technologies are reaching their limits, research into replacing conventional silicon memory devices is being actively done. Next-generation memories are classified into ferroelectric memories, ferromagnetic memories, phase-change memories, nanotube memories, holographic memories, organic memories, polymer memories and the like according to the material constituting a cell which is a basic unit in a semiconductor. Among these memories, a polymer memory realizes memory characteristics by forming a memory layer between upper and lower electrodes using an organic polymer material and then applying a voltage to the memory layer to provide bistability to the resistance value of the memory layer. Here, the cell formed at the place where upper and lower electrodes intersect each other provides bistability. That is, the polymer memory is a memory in which the resistance of the polymer material existing between upper and lower electrodes is reversibly changed by electrical signals to record and read the data “0” and “1”. Such polymer memory is expected to become a next-generation memory that can realize nonvolatility, which is an advantage of conventional flash memories, and that can overcome the problems with processability, manufacturing cost and degree of integration, which are the disadvantages of conventional flash memories.
As examples of organic and polymer memories, US Patent Application Publication No. 2004-27849 proposes an organic memory device in which metal nanoclusters are applied between organic active layers, and Japanese Unexamined Patent Application Publication No. S62-95882 discloses an organic memory device in which CuTCNQ (7,7,8,8-tetracyano-p-quinodimethane) is used as an organometallic complex compound for charge transfer. However, these organic memory devices are problematic in that the manufacturing process thereof is complicated because organic active layers are formed by vacuum deposition, it is difficult to uniformly form metal nanoclusters in the device, the production yield thereof is low, and the manufacturing cost thereof increases. Meanwhile, in a nonvolatile memory device using a polymer, as the compound used to form the active layer, there are a polythiophene-based polymer compound, a polyacetylene-based polymer compound, a polyvinylcarbazole-based polymer compound and the like (refer to the documents [H. S. Majumdar, A. Bolognesi, and A. J. Pal, Synthetic metal 140, 203-206 (2004)]; [M. P. Groves, C. F. Carvalho, and R. H. Prager, Materials Science and Engineering C, 3(3), 181-183 (1995)]; and [Y.-S. Lai, C.-H., Tu and D.-L. Kwong, Applied Physics Letters, 87, 122101-122103 (2005)]).
Here, the polythiophene-based polymer compound is disadvantageous in that the voltage values representing ON/OFF states are high, this polymer compound is unstable in air, and the ratio of ON/OFF is not constant. Further, the polyacetylene-based polymer compound is problematic in that it is difficult to actually form this polymer compound into a memory device because this polymer compound is known as a conjugated polymer which is the most easily oxidized in the air, although it is possible for this polymer compound to be formed into a memory device. Further, it is reported that the polyvinylcarbazole-based polymer compound exhibits excellent switching characteristics, and, currently, research into the polyvinylcarbazole-based polymer compound is being actively done (refer to the document [Y.-S. Lai, C.-H., Tu and D.-L. Kwong, Applied Physics Letters, 87, 122101-122103 (2005)]). Meanwhile, polyaniline has also been used as a raw material of a memory device, but is problematic in that the solubility of polyaniline in an organic solvent is low (refer to the document [R. J. Tseng, J. Huang, J. Ouyang, R. B. Kaner, and Y. Yang, Nano Letters, 5, 1077-1080 (2005)]).